Figure 3. Numerical simulation of Cdc42 polarization under various conditions.

a. Cartoon representation of the relative spatial organization of endocytosis and exocytosis and non-uniform diffusion in the polar cap region. Blue balls represent Cdc42 molecules. Red and green circles depict exocytic (exo) and endocytic (endo) vesicles, respectively. b.-i. Simulations of Cdc42 polarization with assumptions similar to those used previously (b-d)¹⁰ (i.e. uniform diffusion and spatially merged endocytosis and exocytosis) and with geometric factors observed in this study (e-i). Descriptions for each condition are given underneath b-i, including the different diffusion coefficients, exocytic vesicle Cdc42 enrichment, and spatial relationship of endocytosis and exocytosis used in each simulation. Exocytic accumulation refers to the enrichment of Cdc42 on secretory vesicles compared to internal membrane. j.k. Quantification of polarity strength as a function of time (j), defined as the ratio of maximum/minimum Cdc42 concentration over the entire cell periphery, color coded to match the quantification of polarity duration in (k), defined as the time during which polarity strength is above 1.3, for the simulations shown in b-i.